- Patients must have histologically or cytologically confirmed breast cancer. Either the primary breast tumor or the metastasis must overexpress HER2; acceptable methods of measurement of HER2 expression include immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH); tumors tested by IHC must be 3+ positive for HER2 overexpression immunohistochemistry ; tumors tested by FISH must be positive.